The Paris Climate Agreement: Why 2 Degrees C?

On December 12th, 2015, which happens to be my birthday, the world received a historic gift. On this Sunday morning, I reflect on this global step towards good stewardship of the planet, the only one we have to support billions of people. At the Conference of the Parties-21 (COP21) in Paris, a major international agreement supported by nearly 200 countries was offered. Rather than lay out the details and why this one differs significantly from previous efforts, I point you to the very clearly written "explainer" in the New York Times on the implications of the agreement. But, why is 2 degrees C the internationally accepted threshold?

Global Carbon Dioxide Snapshot. Source: NASA Earth Observatory

If you are reading this in the United States, a 2 degrees C change is equal to a 3.6 degrees F change. If you have kids or grandkids, ask yourself what would happen to their body's systems if they ran a continual fever of 102 degrees F. I use this analogy because out of context 1-3 degrees may not sound like much to the average person. Yes, our climate has always changed due to natural variability associated with planet's orbital changes (Milankovitch cycles), solar activity, volcanic eruptions, mountain-building and more. Most climate scientists are well aware of these natural changes. Climate sensitivity is the amount of warming as a function of the greenhouse gas forcing and various processes that I just mentioned while accounting for various feedbacks. There is ongoing climate discussion, as any good science community should, on the level of climate sensitivity. A few scholars argue that the 2 degree number is too aggressive, and others suggest it is too conservative.

Dr. J. Marshall Shepherd, a leading international expert in weather and climate, was the 2013 President of American Meteorological Society (AMS) and is Director of the University of Georgia’s (UGA) Atmospheric Sciences Program. Dr. Shepherd is the Georgia Athletic Associati...